In a semiconductor device manufacturing system, since a serious problem arises in a semiconductor device manufacturing process if oil back diffusion occurs from a pump into a process chamber of the semiconductor device manufacturing system, use has conventionally been made of a so-called dry pump, a mechanical booster pump, a turbomolecular pump, and the like where there is no occurrence of contact between suction gas and oil.
With respect to these dry pump, mechanical booster pump, and screw pump, a problem exists that shaft seals are provided at both ends, i.e. on the suction side and the discharge side, and particularly a seal gas amount of the shaft seal on the suction side and a leakage amount from the seal cause a reduction in pumping speed so that there is no alternative but to use such a pump that has an unnecessarily high pumping speed.
Further, since molecular weights of process gas, carrier gas, gas to be produced, and so on are broad, i.e. from 1 to one hundred and several tens, it is the current situation that the foregoing pumps are properly used depending on their pumping characteristics for those various gases and their inherent pumping regions.
On the other hand, a problem exists that since the pumping speed is lowered depending on the kind of exhaust gas, a pump having a large pumping speed is inefficiently used. Further, with respect to general dry pumps and mechanical booster pumps, there is a problem that product is deposited inside the pump between an inlet port and a discharge port.
Drawbacks of a conventional screw pump will be explained with reference to FIG. 5.
Referring to FIG. 5, in the conventional screw vacuum pump, since a back-diffusion amount from a discharge port and a back-diffusion amount of diluent gas are large, the ultimate pressure becomes about 3 Pa and, as indicated by a curve 2 in FIG. 5, the pumping speed largely decreases on the molecular flow region side. Further, the pumping speed for hydrogen becomes ⅓ to ½ of that for nitrogen and, as indicated by a curve 3 in FIG. 5, since the compression ratio is small, the pumping speed extremely decreases.
Further, since screw engagement of the conventional screw vacuum pump is not located outside gear engagement pitch circles determined by a distance between axes of male and female rotors and the numbers of teeth of the male and female rotors, product generated in the semiconductor device manufacturing process is stuck to screw engagement portions, thereby causing failure.
Therefore, it is an object of this invention to provide a screw vacuum pump that can maintain the stable pumping performance down to about 0.1 Pa regardless of the kind of gas.